I'm not good at picking favorites, so I'm just going to make a list of how enjoyable each planet was:

Korriban: Great backstory. I always enjoy exploring around it as well as the lore. You have to admit, tomb exploring is one of the best experiences in the whole game. BOS:SR just makes it a lot funner.

Taris: I love its complexity - there's just so much to do on it. A shame that the other planets weren't as large. And don't get me started on the Duelling Ring!

Manaan: The music is so orgasmic - I tell you. My favorite ambiance music in the whole game. It brings a much different feel than all the other planets, and the Selkath is a great new species that BioWare introduced into Star Wars.

Dantooine: So peaceful and tranquil - and is home to one of my favorite scenes in the game - creating your lightsaber. Oh, how proud you feel when you have the mystical weapon in your own hands! And lets not forget the Crystal Cave!

Unknown World: Probably the greatest scenery in the whole game. BioWare clearly outdid themselves with the beaches, temples, and the hammerhead cruiser. And the Rakata, oh the Rakata!

Tatooine: I liked Tatooine. As with everything else BioWare does, the expansion on the environment of it - especially the Sand People - is simple awesome. I enjoyed taking the path to learn their culture, and then slaughtering them to get my pearl back!

Kashyyyk: So many trees - and it shows the darker side of Czerka Corp. I liked the Shadowlands - it was dark and scary on my first playthrough - and the introduction of the Ritual Beast scared the hell outta me.

Yavin Station: A bit short for my liking - but the history of Yavin IV and all of the items make up exponentially for it. If only BioWare allowed us to go to Yavin IV.

You just managed to sum up all the awesomeness and the fact that every single place has its very own character and charm in just a few sentences. I agree with every word. (Except for BOS:SR, never got that working :P )